ABS probably *is* activating. Re-check the wiring and connections from the speed sensor in the rear diff all the way through to the CAB (Controller, Anitlock Brake). You'll probably find that the signal voltage is dropping below a threshold, causing the ABS to think the wheels have stopped turning and engage. Check the power connections to the CAB. Also check the ground; there should be a bonding post near the fender. Check the voltage between that bonding post and the battery's negative post; it should be almost zero (maybe a few tens of millivolts).
Search back 8-10 years; you should find a few of my posts about what I found with the Abbott speedometer adjuster, including voltages and thresholds.